The objective of the proposed research is to determine the mechanisms involved in the inhibition of ovulation that occurs during lactation. Concentrations of serum LH, FSH, prolactin, estradiol and progesterone will be estimated by radioimmunologic techniques in order to elucidate the following objectives: 1) to compare the concentrations of these hormones during lactation with other reproductive states, 2) to determine the ability of the hypothalamus, pituitary and ovaries to respond to appropriate stimuli during lactation, 3) to dissociate the effects that neural and hormonal factors resulting from the suckling stimulus have on the hypothalamo-pituitary- ovarian axis and 4) to establish if the steroidal environment during pregnancy is a prerequisite for inhibition of ovarian function to occur during lactation. These studies will provide further information regarding neural and hormonal control of hypothalamic and pituitary regulation of gonadotropin secretion. In addition, the mechanisms involved in the inhibition of cyclicity during lactation may have applications in new methods of birth control.